Role of adult neurogenesis in hippocampal-cortical memory consolidation

Abstract

Acquired memory is initially dependent on the hippocampus (HPC) for permanent memory formation. This hippocampal dependency of memory recall progressively decays with time, a process that is associated with a gradual increase in dependency upon cortical structures. This process is commonly referred to as systems consolidation theory. In this paper, we first review how memory becomes hippocampal dependent to cortical dependent with an emphasis on the interactions that occur between the HPC and cortex during systems consolidation. We also review the mechanisms underlying the gradual decay of HPC dependency during systems consolidation from the perspective of memory erasures by adult hippocampal neurogenesis. Finally, we discuss the relationship between systems consolidation and memory precision.

Figure 1

Adult neurogenesis modulates the hippocampus dependent periods of memory. The vertical axis shows the HPC dependence of contextual fear memory. The horizontal axis shows the days after learning. The HPC dependence of contextual fear memory gradually decays with time in control mice. In contrast, X-ray irradiation or genetic overexpression of follistatin (FSM), both of which severely impair hippocampal neurogenesis, prolong the HPC-dependent periods of contextual fear memory, when compared to control mice. Conversely, the running wheel exercises, which promote hippocampal neurogenesis, speed up the decay rate of the HPC dependency of contextual fear memory, when compared to control mice.

Figure 2

How does adult neurogenesis accelerate the systems consolidation? Adult hippocampal neurogenesis in the DG may help to generate the SPWs in CA3. The SPWs could provide the activation required to drive inter-cortical plasticity through the reactivations of CA1 pyramidal cells, and therefore promote cortical consolidation during subsequent periods of inactivity and sleep. Simultaneously adult neurogenesis erases the pre-exciting memory in the DG.